In any mobile communication system, such as a GSM network, active calls conducted between a mobile station and a base station need to be handed over to a different base station as the mobile station moves between different coverage areas, or cells. Depending on how each cell is defined, handover may require the active call to be re-routed simply through a different base station transceiver BTS, through a different base station controller BSC or through a different mobile services switching center MSC. Handover may also be necessary when capacity problems are met in any one cell.
Handover necessitates a certain amount of operation and maintenance activities on installation of a system, such as defining neighbouring cells, as well as the base station controller BSC and mobile services switching center MSC that controls the cell, defining which cell frequencies should be measured and what threshold value to use to initiate handover. In a conventional GSM network the base station controller BSC sends a mobile station MS a list of predetermined frequencies to be measured. Two lists may be sent out, a first list being used for idle mode, such as when the mobile station MS is roaming, and a second used for active mode when a call is ongoing. This second list defines which frequencies the mobile station MS should measure and report back on. These lists contain a set of values that refer to absolute radio frequency channel numbers ARFCN of neighbouring cells. In addition to these frequency channel numbers the base station controller BSC also knows base station identity codes BSIC of all neighbouring cells. The mobile station MS measures the frequencies defined by these channel numbers and reports these measurements to the base station controller. In practice, the mobile station MS will report on only the six best measurement values and only for those cell frequencies with which that the mobile station can synchronise and consequently receive an identity code relating to the base station (BSIC). The measurement report sent back to the base station controller BSC by the mobile station MS includes a reference to the absolute radio frequency channel numbers ARFCN, the base station identity codes (BSIC) and an indication of the received downlink signal strength. In fact the report does not specify the exact absolute radio frequency channel numbers ARFCN but rather refers to the position this number occupied in the measurement list. On the basis of this report, the base station controller BSC decides whether handover is necessary and to which cell. The initiation of handover is performed according to the standard GSM mechanism for each vendor. Specifically, a message is sent by the base station controller to the mobile services switching center MSC connected to the base station controller BSC indicating that handover is required. This message contains a cell identifier, encompassed in a cell global identity CGI, which defines the mobile country code, mobile network code, location area code and cell identifier for the cell to which handover is requested. The cell global identity CGI is fetched by the base station controller from a list using the base station identification code BSIC and absolute radio frequency channel number ARFCN obtained for the cell. With this cell global identity CGI the mobile services switching center MSC is able to determine which other MSC handles the cell defined by the CGI value.
Recently proposals have been made to extend conventional cellular networks by including access networks that utilise a low power unlicensed-radio interface to communicate with mobile stations. The access networks are designed to be used together with the core elements of a standard public mobile network and consist essentially of plug-in low-power unlicensed radio transceivers, or access points, each designed to establish an unlicensed radio link with a mobile station MS and a controller or interface node connecting the unlicensed radio transceivers with the mobile core network. Suitable unlicensed-radio formats include digital enhanced cordless telecommunications (DECT), wireless LAN and Bluetooth. An adapted mobile handset capable of operating over both the standard air interface (e.g. the Um interface) and the unlicensed-radio interface means that the subscriber requires only one phone for all environments. The access network is constructed so that the core elements, such as the mobile switching centers MSC, of the public mobile network views the interface node as a conventional base station controller BSC. Such an access network and the mobile station for use with this access network is described in European patent application No. EP-A-1 207 708. The content of this application is incorporated herein by reference.
The low power and resultant low range of the unlicensed-radio interface means that several such access networks may be provided in relatively close proximity, for example one access network per floor of an office building or in a private home. The connection between the unlicensed-radio tranceivers and the associated controller is provided by a fixed broadband network. Preferably communication over this network uses the internet protocol IP, which greatly facilitates the installation of the access network, permitting a subscriber to plug-in an unlicensed-radio transceiver in his own home and consequently install an unlicensed-radio access point himself. However, the flexibility of such unlicensed-radio access networks also presents difficulties. Since an access point can be freely installed and moved by a subscriber to a separate city, state or even country, yet still connect to its original access network controller, the exact location of the access point cannot be tracked by the core network. This imposes huge demands on the operation and maintenance activities required for handover to and from the unlicensed-radio access network, as neighbouring cells may change frequently. Also billing restraints in some areas may require the re-assignment of a relocated access point to a more appropriate access controller, particularly if revenue from calls originating from a specific access point must be accounted for in a specific region of a country. In view of the small size of the access points to an unlicensed radio access network, it would not be cost effective for operators to configure each access point separately. However, it is also undesirable to leave the task of configuration to the subscriber as such a solution would be error prone and consequently unreliable. In addition suppliers would not wish to configure access points differently depending on where these are ultimately to be installed.